1. Field of the Invention
The present invention relates to a thin film, a method for manufacturing the thin film, and an electronic component. More particularly, the present invention relates to a thin film, for example, a piezoelectric material thin film, a dielectric material thin film, and a magnetic material thin film, or other suitable thin film, used for an electronic component, for example, a piezoelectric thin film resonator, a filter, sensor, and an actuator or other suitable device.
2. Description of the Related Art
Since a resonant frequency of a piezoelectric resonator using a thickness longitudinal vibration of a piezoelectric substrate is inversely proportional to the thickness of the piezoelectric substrate, in the superhigh frequency regions, the piezoelectric substrate must be processed extensively to be very thin. Regarding the decrease in the thickness of the piezoelectric substrate itself, however, in the fundamental mode, several 100 MHz have been believed to be the practical limit of the high frequency due to the mechanical strength and restrictions in the handling thereof.
In order to solve the aforementioned problems, diaphragm type piezoelectric thin film resonators have been suggested, and have been used for filters and resonators. FIG. 5 is a sectional schematic diagram of an example of a conventional piezoelectric thin film resonator. The piezoelectric thin film resonator 1 as shown in FIG. 5 includes a Si substrate 2. A thin film support member 3 having a thickness of several μm or less is formed on the substrate 2 by a partial etching from the reverse surface using a micromachining method. An AlN piezoelectric thin film 5 is provided on the thin film support member 3 as a piezoelectric material thin film including a lower layer electrode 4a and an upper layer electrode 4b on both major surfaces as a pair of excitation electrodes. The thin film support member 3 and peripheral portions thereof thereby define a diaphragm 6. In the piezoelectric thin film resonator 1 as shown in FIG. 5, the thin film support member 3 can be formed to be thin using the micromatching technique, and the AlN piezoelectric thin film 5 can be also formed to be thin by the sputtering, or other suitable process, so that the high frequency characteristics may be extended to several 100 MHz to several 1,000 MHz.
In order to produce resonance characteristics that are superior in the temperature characteristics of the resonant frequency and in the antiresonance characteristics, a piezoelectric thin film resonator as shown in FIG. 6 has been suggested. FIG. 6 is a sectional schematic diagram of another example of a conventional piezoelectric thin film resonator. When the piezoelectric thin film resonator 7 as shown in FIG. 6 is compared with the piezoelectric thin film resonator 1 as shown in FIG. 5, a thin film 8 made of SiO2, Si3N4, Al2O3, or ZnO, is formed as the upper layer of the Si substrate 2.
In order to realize the piezoelectric thin film resonators 1 and 7 as shown in FIG. 5 and FIG. 6, the stress of the entire device must be controlled so as not to destroy the diaphragm 6. Since the thin film of SiO2 has strong compressibility, the thin films of Si3N4 and Al2O3 have a strong tensile property, and so forth, the internal stress of the AlN piezoelectric thin film 5 must be controlled.
It is said that in order that excellent piezoelectricity to be achieved in the AlN piezoelectric thin film, the C axis is preferably oriented in the direction that is perpendicular to the substrate, and the half-width of the rocking curve is preferably small, as described in, for example, “Acoustic Wave Technology Handbook” written by the Japan Society for the Promotion of Science, 150th Committee on Acoustic Wave Technology issued by Ohmsha, Ltd., 1991. In general, in the case in which the AlN piezoelectric thin film is formed by the sputtering method, an excellent C axis preferred orientation film is produced at a low gas pressure region, that is, at a film generating a pressure of 0.6 Pa or less, as described in, for example, J. Mater. Res., Vol. 12, No. 7, p. 1850 (1997) by A. Rodriguetz-Navarro, W. Otano-Rivera, J. M. Garcia-Ruiz, R. Messier, and L. J. Pilione. On the other hand, since the peening effect is strong at the low gas pressure region, the formed AlN piezoelectric thin film has strong compressibility. Therefore, the internal stress of the AlN piezoelectric thin film has previously been controlled with the gas pressure during the film formation.
When the gas pressure during the film formation is increased, however, the preferred orientation property of the AlN piezoelectric thin film is reduced, and accompanying this, the resonance characteristic is also reduced. As a consequence, the internal stress of the AlN piezoelectric thin film controlled by the gas pressure and the preferred orientation property have a trade-off relationship, so that controlling the internal stress while keeping the preferred orientation property high was not possible.